1. Field of the Invention
The present invention relates to a method and an apparatus for reproducing a carrier, and more particularly, to a method and an apparatus for reproducing, from a receive signal whose carrier is phase-modulated, the carrier.
2. Description of the Background Art
In receiving a signal which is phase-modulated as in phase shift keying (PSK) and demodulating the phase-modulated signal, a carrier must be produced from the phase-modulated signal. The reproduced carrier is used for examining the phase of a receive signal.
As a conventional carrier reproducing device, one described in Japanese Patent Publication No. 19666/1990, for example, has been known. FIG. 14 is a block diagram showing the construction of the conventional carrier reproducing device. In FIG. 14, the conventional carrier reproducing device comprises a receive signal input terminal 21, multipliers 22, 23 and 27, a .pi./2 radian phase shifter 24, low-pass filters 25 and 26, a phase comparator 28, a loop filter 29, a voltage controlled oscillator 30, level judging units 31 and 32, square units 33 and 34, and an adder 35. FIG. 14 illustrates the construction in the case of a binary PSK system.
Description is now made of operations performed by the carrier reproducing device shown in FIG. 14. A receive signal which is phase-modulated is inputted from the receive signal input terminal 21. The receive signal is fed to the multipliers 22 and 23. The multiplier 22 multiplies the receive signal by an output signal of the voltage controlled oscillator 30 whose phase is shifted 90.degree. by the .pi./2 radian phase shifter 24. The multiplier 23 multiples the receive signal by the output signal of the voltage controlled oscillator 30. The receive signal is converted into complex base band signals by the multipliers 22 and 23.
An output of the multiplier 22 is then fed to the multiplier 27 as a signal 501 representing a sine component of the receive signal after its extra noise component is removed by the low-pass filter 25. An output of the multiplier 23 is fed to the multiplier 27 as a signal 601 representing a cosine component of the receive signal after its extra noise component is removed by the low-pass filter 26. The multiplier 27 multiplies the signals 501 and 601 together, to output a signal 701 having a sine component of twice the difference between the phase of the receive signal and the phase of the voltage controlled oscillator 30.
Furthermore, the output of the multiplier 22 is squared by the square unit 33 after the extra noise component is removed by the low-pass filter 25, and the squared output is fed to the adder 35. Further, the output of the multiplier 23 is squared by the square unit 34 after the extra noise component is removed by the low-pass filter 26, and the squared output is fed to the adder 35. The adder 35 adds the output signals of the square units 33 and 34 and synthesizes the result, to output a signal 801 having a cosine component of twice the difference between the phase of the receive signal and the phase of the voltage controlled oscillator 30. The signals 701 and 801 are fed to the phase comparator 28 after the respective levels are judged by the level judging units 31 and 32, so that an error between the phase of the receive signal and the phase of the voltage controlled oscillator 30 on a complex plane is detected. An output of phase comparator 28 controls the oscillation phase of the voltage controlled oscillator 30 via the loop filter 29.
In the above-mentioned construction, an output signal of the voltage controlled oscillator 30 at the time point where the error between the phase of the receive signal and the phase of the voltage controlled oscillator 30 reaches zero becomes a reproduced carrier. That is, the phase of the output signal of the voltage controlled oscillator 30 at this time point coincides with the phase of the receive signal. The reproduced carrier is fed to a demodulating circuit (not shown), and is used for examining the phase of the receive signal (0.degree. or 180.degree. in the case of the binary PSK system).
An example corresponding to a quadrature PSK system by further adding a multiplier, a square unit and an adder to the above-mentioned construction is also described in the above-mentioned gazette.
As described in the foregoing, in the conventional carrier reproducing device, both a sine component and a cosine component are obtained with respect to the phase difference. This is for preventing a so-called hang-up phenomenon. The hang-up phenomenon occurs when a reproduced base band signal is just shifted +90.degree. or -90.degree. from its ideal phase position. In this case, it cannot be judged whether the base band signal should be rotated in a positive direction or a negative direction. In the conventional carrier reproducing device, both the sine component and the cosine component of the phase difference are obtained, and hysteresis characteristics are introduced when the direction of rotation is determined, so that the direction of rotation is determinable when the base band signal is shifted +90.degree. or -90.degree. from the ideal phase position.
As described in the foregoing, in the conventional carrier reproducing device, a lot of multipliers and square units must be provided in order to prevent the hang-up phenomenon, so that the circuit scale thereof is increased. Particularly when an input signal takes a digitized numerical value, and the carrier reproducing device is composed of a digital circuit, the scales of the multiplier and the square unit are large, so that the scale of a portion where the phase difference is detected is significantly increased.